anderson



Jan. 24, 1956 Filed June 15. 1950 W. T. ANDERSON, JR., ET AL VAPOR DISCHARGE DEVICE 2 Sheets-Sheet l INVENTORS WILLIAM T. ANDERSON,JR

FR NKLlN CLARK 4 TTOQNE) Jan. 24, 1956 w. T. ANDERSON, JR., ET AL 2,732,513

VAPOR DISCHARGE DEVICE Filed June 13 1950 2 SheetsSheet 2 ilZ].3n 50 LAMP VOL T5 Q i I i I T/ME' /N MINUTES FROM IGNITION OF DISCHIQRGF IGNITION VOL7'5 TIME MINUTES pmcs/vr 1w woes/v 645 40050 7'0 10mm nesolv e45 ATTORNEY United States Patent VAPOR DISCHARGE DEVICE William T. Anderson, Jr., Maplewood, and Franklin Clark, Chatham, N. J., assignors to Hanovia Chemical and Mfg. Company, Newark, N. J., a corporation of New Jersey Application June 13, 1950, Serial No. 167,866 1 Claim. (Cl. 313-185) The present invention deals with an electrical discharge device and more particularly with a metal vapor discharge device.

Unlike tungsten filament incandescent lamps which attain full brilliancy of light emission almost instantly on application of the power, metal vapor discharge devices do not usually reach full light output until after the vaporizable metal contained therein has been evaporated, which in many instances requires a number of minutes from the initial ignition of the device.

The warm-up time of a metal vapor discharge lamp is one of the most important factors in determining the practical application of such a lamp for specific uses, since many lamp installations can use this type of light producing device only if the warm-up time can be maintained below a certain maximum.

Various means have been proposed for shortening the warm-up time of metal vapor lamps, e. g. means for elevating lamp starting potentials by modifications in lamp circuits, modifications in lamp dimensions, etc., whereby lamp warm-up was accelerated but with an attendant decrease in the useful life of the lamps.

It is an object of the present invention to provide a metal vapor discharge device characterized by a substantially short warm-up period and a long useful life. It is another object of the present invention to provide a metal vapor discharge lamp having a long life and a short Warm-up period, and which incorporates an economical, convenient and simple means for attaining such advantages. It is a further object of the present invention to provide a metal vapor discharge device having a wide field of commercial applications including fields where otherwise incandescent lamps and carbon arc lamps are used. Other objects and advantages of the present invention will become apparent from the descriptionhereinafter following and the drawings forming part hereof, in which:

Figure 1 is a diagrammatic representation of a metal vapor discharge device according to the present invention including operating circuit therefor,

Figure 2 is a schematic illustration of a metal vapor discharge lamp according to the present invention,

Figure 3 is a graphical comparison of a lamp according to the present invention and a conventional lamp, and

Figure 4 is a graphical representation of the warmup time for a lamp according to the present invention and the starting potential associated therewith.

Referring to Figures 1 and 2, the invention is concerned with a metal vapor arc lamp 1, e. g. a mercury vapor lamp, having a tubular or elongated light transmissive envelope 2 containing spaced electrodes 3 and 4, and an ionizable medium. The envelope 2 contains suflicient Vaporizable metal 5 to provide, when completely vaporized, a metal vapor pressure of at least one-quarter atmosphere or more, i. e. such pressures as are char acteristic of high pressure lamps, and a starting gas for ignition of a gaseous electrical discharge between the electrodes, when a suitable voltage is applied, to vaporize said metal to provide the desired metal vapor for the metal vapor discharge. The electrodes are solid coils or masses of a refractory metal such as tungsten, tantalum, nickel, etc., and may or may not have associated therewith other metals such as thorium, zirconium, barium, etc., or oxides of such metals or both, to assist in starting the discharge when ignition voltage is applied and to extend the life of the electrodes at the operating voltages and currents. The ignition voltage and the lamp operating voltage are applied to the electrodes of the lamp by, for example, the circuit arrangement illustrated in Figure l, which is more particularly described in United States Patent No. 2,482,894. Such a circuit comprises supply lines 6 and 7 of alternating current, a reactive transformer 8 having a primary winding 9, a

secondary winding 10, a leakage reactance 11, and in said circuit a series capacitor 12 between the lamp 1 and secondary of the transformer.

The watts consumed within a lamp by the starting gas discharge and by the electrodes warm the lamp and vaporize the metal condensed on the wall of the lamp envelope. The rate at which the lamp warms up is dependent upon many factors, such as watts per unit volume of envelope, heat radiation from the arc and convection cooling by ambient air currents. Thus, lamp dimensions or the employment of an additional outer envelope to shield the lamp proper from convection air currents have very definite, influence on the warm-up rate. However, there are practical limitations on the size of the lamp envelopes which are intimately related to lamp life. For example, a commercial mercury vapor lamp of the type which has a small envelope and operates at very high wattage per unit volume of envelope has a useful life of only about 50 hours, whereas a commercial mercury vapor lamp with a much larger envelope and operating at the same wattage input as the smaller lamp, e. g. a wattage input of about 1000 watts, has a useful life of several thousands of hours. The warm-up rate of the smaller envelope lamp is less than about onetenth of a minute while that of the larger envelope lamp is several minutes since it has a lower wattage per unit volume.

The provision of an additional outer lamp envelope for the conservation of heat is common practice. Its employment has the obvious disadvantage, however, that reflection and absorption losses by the vitreous components reduce the light output and efiiciency of the lamp. For visible light, these-losses amount to about 12 percent and for ultraviolet radiations the losses may be as high as 40 percent.

The present invention overcomes the disadvantages of conventional practices in that lamps having substantially large envelopes are rendered capable of warming up in a comparatively short time without requiring the provision of additional lamp envelopes. However, lamps. which are provided with outer envelopes have a still further reduction in warm-up time by the employment of the invention. The invention is concerned particularly with increasing the wattage consumption in the starting gas discharge after ignition of the lamp, whereby the heat available for the vaporization of the vaporizable metal is increased. The increase in starting gas wattage is accomplished by the addition of a second gas to the main starting gas. For use as a main starting gas employed in accordance with this invention, the lamp envelope contains at least one of the gases of the group consisting of argon, krypton and xenon. The combination of the second gas and at least one of the gases of the above group has a higher electrical resistance for the discharge after ignition than the main starting gas alone, but only a relatively does not decrease thetuseful life of discharge lamps.

A consideration of the gases used as starting gases in accordance with this invention is deemed essential to establish and clarify the basis of the present invention.

The acceleration and momentum with which gas ions in an electric field strike the walls of the lamp envelope and electrodes is related to the mean free path, i. e. the average distance travelled by a gas particle before collision, which has a definite influence on the life of a vapor discharge lamp. An increase in mean free path of gas particle for the same electric field results in increased rate of deterioration of the lamp and shortened lamp life because the ions are accelerated over a longer distance before stopped by collision. Therefore, when they strike envelope walls and electrodes they do so with greater momentum and, consequently, cause greater damage. The mean free path of gases varies according to the particular gas and gas pressure, i. e. the mean free path of a given gas will decrease as its pressure increases. It is apparent from the above that a lamp will have a longer useful life if the lamp envelope contains a gas of substantially short mean free path. Nevertheless, the criterion of short mean free path is not alone sufiicient to render the gas applicable as a starting gas according to the present invention, since, in addition to a short mean free path, the gas must also have substantially low ionization voltage and a small high speed ion range factor. This latter consideration is a convenient means for comparing the average distance through which an ion can maintain a velocity suihciently high to produce damaging effects and is numerically equal to the product of the initial energy of the ion raised to the 3/2 power and a constant which is characteristic of each individual gas. A gas of the group consisting of argon, krypton and xenon meets such requirements but does not substantially increase the warm-up rate of a vapor discharge lamp unless its pressure is increased to above about millimeters for krypton and xenon and millimeters for argon and such an increase in pressure raises the ignition voltage to impractical heights.

However, in accordance with this invention, we have found that the advantages of the main starting gas above described can be maintained, excluding impractically high ignition voltages, when a particular second gas is mixed therewith to render the lamp capable of increased wattage consumption after ignition so that it becomes unnecessary to raise the starting gas pressure to increase the warm-up rate of the metal vapor discharge lamp. The particular second gas herein referred to is nitrogen.

Regarding the gases considered in this invention, the values for mean free path at normal temperature and pressure, ionization volts and high speed ion range factor are as follows:

A gas of the group consisting of argon, krypton and xenon may be used as a starting gas when substantially long warm-up periods are not objectionable.

Test lamps made with a nitrogen filling instead of, for example, argon, proved to be impractical because its employment resulted in a high ignition voltage. For example, with a barium on tungsten electrode the cathode drop voltage for argon is 93 volts whereas for nitrogen the voltage drop was found to be 157 volts at an electrode temperature of 25 C.

A mixture of nitrogen and argon as a starting gas has a very pronounced effect on the warm-up of metal vapor discharge lamps as illustrated by Figure 3, which shows the effect on the warm-up rate of the addition of only 0.018 millimeters pressure nitrogen to 10 millimeters argon in a typical linear high pressure mercury discharge lamp. The time to attain complete warm-up (curve a), i. e. full brilliancy, is that at which steady state lamp voltage was reached and, in this case, such voltage was 550 volts. The lamp was operated at substantially constant current. It is evident from the illustration that the nitrogen addition decreased the time of warm-up to less than one-half of the normal rate (curve b), i. e. the rate of warrnu p was more than doubled. The lamp utilized had an arc length of 48 inches and employed mercury vapor at a pressure of about 0.4 atmosphere when the lamp was completely warmed and all mercury evaporated.

Figure 4 illustrates the time to attain complete warm-up and also the ignition potential associated with the various percentages of nitrogen gas added to argon at 10 millimeters pressure. Variations from the curves shown are evidenced when nitrogen in the same percentages is added to krypton or xenon or mixtures thereof with or without argon and when pressures other than 10 mm. of argon are'used. Such variations include lower ignitionvoltages with an increase in warm-up time. However, such variations are within the scope of the invention in that the life of the lamp is not affected, the ignition voltages are maintained substantially low, and rate of lamp warm-up is substantially equal to that of an argon-nitrogen filling when the nitrogen component is slightly greater.

It is apparent that the present invention concerns a starting gas for metalvapor discharge devices, said starting gas being a mixture of nitrogen andat least one gas of the group consisting of argon, krypton and xenon and, for a warm-up time under about ten minutes, according to Figure 4, the nitrogen constitutes from about 0.03% to about 0.4% of the mixture and said warm-up time decreasing at a rapid rate with the addition of up to 0.3% nitrogen. The preferred range of the nitrogen constituent is from about 0.1% to about 0.2% when mixed with argon and from about 0.2% to about 0.3% when mixed with krypton and from about 0.3 to about 0.4% when mixed with xenon. For a-mixtureofkrypton and xenon, the nitrogen is preferably between 0.1% and 0.3%.

The above ranges of the nitrogen component exclude traces of nitrogen since desirable electrical resistance of the starting gases are not sufficiently provided to decrease the warm-up ratebelow about ten minutes with mere traces of nitrogen. At percentages of nitrogen higher than about 0.3%, there is no further appreciable gain in.

warm-up rate andthe ignition voltage becomes too high for practical use.

The present invention is not to be limited to the specific values and structures, etc., herein used as examples, since the scope of the invention includes lamps having different are lengths, lamps in which cadmium is the'vaporizable metal as well as mercury, lamps characterized by high mercury vapor pressures such as super pressure lamps,

and various starting gas pressures, e. g. 20 millimeters,

pressure, to which nitrogen within the limits indicated is added as a part of the starting gas comprising at least one of the gases of the group herein described. The invention may be employed with conventional metal vapor arc devices in which case the time. for warming-up such lamps is decreased to obtain full operational brilliancy. Also, while we have described the invention with particular reference to a lamp with a single discharge envelope, it will be understood that the invention applies to any metal vapor lamp, preferably high pressure mercury or the like vapor lamps with or without auxiliary envelopes or the like vapor lamp with-or without auxiliary starting circuits.

What we claim is:

A high pressure vapor ele ctric gaseous .discharge are device comprising a light transmissive envelope containing spaced electrodes connective to a source of voltage, a vaporizable metal and a starting gas composed of a mixture of nitrogen-and jargon within said: envelope, said nitrogen constituting from about 0.1% to about 0.2% t

References Cited in the file of this patent UNITED STATES PATENTS v Miesse Oct. 23, 1934 6 Scott May 26, 1936 Swanson Apr. 18, 1939 Davies Mar. 10, 1942 Kern Mar. 10, 1942 Mager Apr. 29, 1947 Noel Apr. 19, 1949 Overbeek July 19, 1949 

